The application relates generally to blowing and reading fuses formed within integrated circuits, and relates particularly to reading the fuses in a way that has improved reliability.
In many systems it is desirable to have a nonvolatile memory. In some applications a static RAM with a backup battery will be suitable, and in other applications an EEPROM may be suitable. But in recent times there has been great pressure on system designers to reduce the size and power consumption of a system. Such pressure is particularly great with cellular telephones, personal digital assistants (PDAs), and notebook computers. In such a system, any decision that adds to the bulk or weight is undesirable, for example a decision to increase the chip count (to accommodate an external memory) or a decision to add a backup battery for a memory.
The general goal of reducing chip count provides motivation to try to use nonvolatile memory devices which can be fabricated within some existing chip in the system. One technology for such memory is the use of fuses. One or more fuses are fabricated within the chip. Circuitry is provided which will pass some specified current through the fuse, which “blows” the fuse. Other circuitry measures the resistance of the fuse. If the resistance is low, then it is presumed that the fuse has not been blown. If the resistance is high, then it is understood that the fuse has been blown. Each fuse represents one binary data bit, and its blown or non-blown state is defined as a logic level of the binary data bit.
Several technologies for on-chip fuses have been proposed and used. One commonly used technology is a polysilicon fuse. A high current is passed through the fuse which “blows” the fuse. As will be appreciated, it is necessary to distinguish later between the “blown” and “non-blown” conditions of the fuse.
It is desirable that the fuse, in its unblown state, have a low impedance, because this makes it easier to get sufficiently high current through the fuse to blow it. It is also desirable that the fuse be constructed in a way that minimizing explosive decomposition which could result in damage to adjacent or nearby circuit structires.
Where a CMOS process is being used to fabricate the chip, to reduce the resistance of the polysilicon fuse in its non-blown state, the fuse is manufactured with Ti-silicide doping. The Ti-silicide doping has the advantage not only of reducing the unblown impedence, but also of minimizing or eliminating explosive decomposition of the fuse.
While Ti-silicide doping has the advantage of reducing the unblown resistance and resisting explosive decomposition, it has a potential drawback in that it “blowing” such a fuse results only in a process of silicide agglomeration once the fuse heats up to around 800° C. This results in the fuse undergoing a phase change of state which results in the fuse impedance changing from its unblown state of around 100 ohms to its blown state of any value from around 1 KΩ to 500 KΩ. This means that it is not easy to know exactly what resistance in the fuse represents a “blown” state.
A typical way of reading the fuse (determining whether it is blown or unblown, that is, whether it represents a binary “1” or binary “0” value) is to compare the value of the fuse impedance with a fixed reference resistance. If the fuse impedance is greater than the reference then it is read as blown, otherwise it is read as unblown. The comparison can be achieved by a number of techniques, but the most obvious is to force identical currents through the fuse and the reference resistor and to compare the resultant voltages using some form of voltage comparator.
Experience shows that such a blown fuse may change its resistance slightly with temperature or aging. This means that if a blown fuse impedance is very close to the reference resistance then it may read as blown one time then as unblown another time. Such a result is extremely undesirable.
It will be appreciated that while this problem is described in particular connection with polysilicon fuses, it presents itself with any fuse technology in which a blown fuse does not blow to a very high impedence.
It would be advantageous if an approach could be developed which permits determining, before a chip is placed into service, whether the chip is likely to provide a reliable reading of its fuses. If such an approach were available, it could be used to test a chip; if the chip turned out to be unlikely to provide reliable reading of its fuses, a decision could be made not to place the chip into service.